Scope default category order to x-axis only

Address review on #5776: y categoricals stay untouched so the
autorange=reversed pattern from doc/python/gantt.md keeps listing
tasks top-down instead of double-reversing.

Adds guard test that px.timeline and horizontal px.bar with color emit
no y categoryarray, while x data-order fix for #3198/#3291 is kept.

Author: vineoy

CHANGELOG.md

@@ -7,7 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#3441](https://github.com/plotly/plotly.py/issues/3441)], with thanks to @hb1915 for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5765](https://github.com/plotly/plotly.py/issues/5765)], with thanks to @Irahan2 for the contribution!
-- Fix Plotly Express scrambling the order of categorical axes when the data is split into several traces (e.g. by `color`); the axis now follows the order of first appearance in the underlying data [[#3198](https://github.com/plotly/plotly.py/issues/3198), [#3291](https://github.com/plotly/plotly.py/issues/3291)]
+- Fix Plotly Express scrambling the order of the categorical x-axis when the data is split into several traces (e.g. by `color`); the x-axis now follows the order of first appearance in the underlying data [[#3198](https://github.com/plotly/plotly.py/issues/3198), [#3291](https://github.com/plotly/plotly.py/issues/3291)]
 
 
 ## [7.1.0] - 2026-09-15

plotly/express/_core.py

@@ -2531,31 +2531,36 @@ def get_groups_and_orders(args, grouper):
 
 def _default_category_orders_for_axes(args, groups, orders):
     """
-    Add data-appearance orders for categorical x/y columns.
+    Add data-appearance order for the categorical x column.
 
     When the data is split into several traces (e.g. by `color`), plotly.js
     orders categories by first appearance across traces, which can differ from
     the order in the underlying data. Recording the data-appearance order here
     lets `set_cartesian_axis_opts` emit an explicit `categoryarray`, matching
     the documented behavior that Plotly Express lays out categorical data in
     the order in which it appears in the data. See #3198.
+
+    Only the x axis is handled: y categoricals are intentionally left alone
+    because `set_cartesian_axis_opts` reverses y orders for top-down display
+    and figures such as `px.timeline` / horizontal `px.bar` pair that with
+    `autorange="reversed"` (see `doc/python/gantt.md`), so an explicit y
+    `categoryarray` would double-reverse and flip the documented order.
     """
     if len(groups) <= 1:
         return orders
     df = args["data_frame"]
-    for letter in ("x", "y"):
-        col = args.get(letter)
-        if not isinstance(col, str) or col in orders or col not in df.columns:
-            continue
-        uniques = df.get_column(col).unique(maintain_order=True).to_list()
-        categories = [
-            value
-            for value in uniques
-            if value is not None
-            and not (isinstance(value, float) and math.isnan(value))
-        ]
-        if categories and all(isinstance(value, str) for value in categories):
-            orders[col] = categories
+    col = args.get("x")
+    if not isinstance(col, str) or col in orders or col not in df.columns:
+        return orders
+    uniques = df.get_column(col).unique(maintain_order=True).to_list()
+    categories = [
+        value
+        for value in uniques
+        if value is not None
+        and not (isinstance(value, float) and math.isnan(value))
+    ]
+    if categories and all(isinstance(value, str) for value in categories):
+        orders[col] = categories
     return orders
 
 

tests/test_optional/test_px/test_px.py

@@ -393,6 +393,25 @@ def test_no_default_category_array_for_single_trace_or_numeric():
     assert fig["layout"]["xaxis"]["categoryorder"] is None
 
 
+def test_no_default_category_array_for_y_with_color():
+    # https://github.com/plotly/plotly.py/pull/5776#pullrequestreview-5478271667
+    # y categoricals must stay untouched so the `autorange="reversed"`
+    # pattern from doc/python/gantt.md keeps listing tasks top-down.
+    df = pd.DataFrame(
+        [
+            dict(Task="Job A", Start="2009-01-01", Finish="2009-02-28", Resource="Alex"),
+            dict(Task="Job B", Start="2009-03-05", Finish="2009-04-15", Resource="Alex"),
+            dict(Task="Job C", Start="2009-02-20", Finish="2009-05-30", Resource="Max"),
+        ]
+    )
+    fig = px.timeline(df, x_start="Start", x_end="Finish", y="Task", color="Resource")
+    assert fig["layout"]["yaxis"]["categoryorder"] is None
+    assert fig["layout"]["yaxis"]["categoryarray"] is None
+    fig = px.bar(x=[1, 2, 3, 4], y=list("abcd"), color=list("ppqq"), orientation="h")
+    assert fig["layout"]["yaxis"]["categoryorder"] is None
+    assert fig["layout"]["yaxis"]["categoryarray"] is None
+
+
 def test_permissive_defaults():
     msg = "'PxDefaults' object has no attribute 'should_not_work'"
     with pytest.raises(AttributeError, match=msg):